Themes for notebook

You can now add full new themes to Sage. From light css touches to full template system replacement. I have used `flask_themes2`.

I have tried to keep changes in the sources at minimum, but jinja2 initialization process had to be modified, and as consequence the rendering of worksheet templates.

Several test themes are included in this branch. The are to locations where themes can be placed. One place is a special new directory `sagenb/sagenb/themes`, intended for providing official distribution themes and/or system wide available themes. The other place is a directory `themes` in `SAGENB_DOT`. Some documentation was added in the help menu. For more information about how to write themes, see `flask_themes2` docs.
Warning `flask_themes` is broken and is incompatible with newer versions of flask.

You must make `flask_themes2` available in the local sage's python path in order to test this feature. An appropriate dependency was added in setup.py.

I know that a new UI redesign is coming. Could this extension make this transition smoother?